1. Field of the Invention
The present invention provides a novel initiator system containing vanadium for the (co)polymerisation of isoolefins, a process for the production of the novel initiator system and the use thereof for the production of polymers from isoolefins optionally with monomers copolymerisable with isoolefins, in particular for the production of butyl rubbers.
2. Description of the Prior Art
The process currently used to produce butyl rubber is known, for example, from Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993. In the slurry process, isobutene is cationically copolymerised with isoprene with methyl chloride as the process solvent using aluminium chloride as the initiator with the addition of small quantities of water or hydrogen chloride. It is also known to initiate this copolymerisation using a combination of tertiary halides combined with Lewis acids (Kennedy, Marechal, Carbocationic Polymerisation, Wiley 1982). A feature common to both processes is that polymerisation temperatures of approximately -100.degree. C. are necessary in order to achieve a molecular weight which is sufficiently high for use in the rubber-processing industry, which entails very costly cooling of the reaction. In these processes, as a general rule molecular weights become ever lower as temperature increases.
It is also known to copolymerise isobutene with various comonomers at temperatures of approximately -40.degree. C. using vanadium tetrachloride as the initiator, either in pure form or as a solution in heptane, with optional activation by light or the addition of aromatic compounds (Miroslav Marek et al., J. Polym. Sci. Polym. Chem. Ed. 16, 2759-2770 (1978); J. Pilar, L. Toman, M. Marek, J. Polym. Sci. Polym. Chem. Ed. 14, 2399-2405 (1976); L. Toman, M. Marek, Makromol. Chem. 177, 3325-3343 (1976); M. Marek et al., U.S. Pat. No. 3,997,417; L. Toman, M. Marek, J. Macromol. Sci.-Chem., A15(8), 1533-1543 (1981); M. Marek, J. Polym. Sci. Symp. 56, 149-158 (1976)). While the copolymers produced using this process do indeed have a rubbery consistency, they are insoluble due to their elevated gel content and precipitate during polymerisation in an alkane solution or in bulk, so causing serious problems for the industrial performance of this reaction.
The homopolymerisation of isobutene with vanadium tetrachloride as the initiator and ammonia (L. Toman, M. Marek, Polymer Bull. 6, 570-576 (1982)) as the co-initiator is described in bulk and in solution (solvent: heptane). Polymerisation was performed in the temperature range from -10.degree. C. to -75.degree. C. in darkness. Polymerisation does not proceed in this process without the presence of ammonia. Yields are dependent upon the molar ratio of vanadium tetrachloride to ammonia and reach a maximum at a ratio of 1:1. The molecular weights achieved are independent of this ratio and rise with increasing monomer concentration. A molecular weight (determined viscosimetrically) of 2015 kg/mol was achieved in bulk polymerisation at -75.degree. C. In heptane at a monomer concentration of 4.65 mol/l, the value was only half so high. The molecular weights achieved decrease as temperature rises. Other polymer characteristics, in particular the gel content, are not stated.
The polymerisation of olefinic hydrocarbons with vanadium tetrachloride in combination with the most varied aromatic compounds as coinitiators in the temperature range from -110.degree. C. to +110.degree. C. is known from U.S. Pat. No. 3,326,879. A disadvantage of the processes described in the stated US patent is that some of the listed coinitiators, if used in a quantity of greater than 100 mol. %, inhibit polymerisation and, as our own tests have shown, the only copolymers of isobutene with isoprene which are obtained are those having an elevated gel content. This also applies to the process described in U.S. Pat. No. 3,997,417 for the polymerisation and copolymerisation of monomers with olefinic double bonds in the presence of polyvalent metal halides, for example vanadium tetrachloride, in the presence of light in the temperature range from -140.degree. C. to +30.degree. C. Thus, according to Example 12, copolymers of isobutylene and isoprene are obtained which have a gel content of 15% and, according to Example 11, copolymers of isobutylene and butadiene which have a gel content of 10%. Depending upon the reaction conditions, molecular weights are between 75 and 550 kg/mol. The polymerisation process described in U.S. Pat. No. 3,998,713 using tetravalent metal halides in combination with alkaline earth or alkali metals or the hydrides or amalgams thereof as co-initiators with irradiation in the temperature range from 0.degree. C. to -140.degree. C. is also unsuitable for polymerising or copolymerising monoolefinic and diolefinic compounds. Here too, an elevated gel content is obtained on polymerisation. Even at low temperatures, the molecular weights of the polymers are unsatisfactory. The same also applies to the polymerisation and copolymerisation of monomers having olefinic double bonds in the presence of, for example, halides of tetravalent vanadium with or without light described in DE 2 125 800 and DE 2 119 305. The achieved molecular weights of the polymers are unsatisfactory and the gel content of the polymers, for example when butyl rubbers are produced, is too high.
It has now surprisingly been found that the above-stated disadvantages during the (co)polymerisation of isoolefins can be avoided if isoolefins are copolymerised in the presence of the initiator system containing vanadium described below.